It has long been the practice in the art to impregnate or otherwise distribute active catalytic metals upon support materials having desired properties of porosity, surface area, thermal and mechanical stability, and suitably inert chemical properties. All of these characteristics of the support are interrelated and contribute in an often unpredictable manner to the ultimate activity of the final catalyst in its intended use. Much effort has been devoted in recent years to the finding of a support material which will withstand the severe mechanical and thermal stresses encountered in catalytic converters for the conversion of nitrogen oxides, carbon monoxide, and unburned hydrocarbons in automobile exhaust gases. Some materials, e.g., alpha alumina, are suitably inert and stable, but in general do not give a final catalyst having the volumetric activity that can be obtained from the same quantity of active metal or metals supported on other less stable supports such as gamma alumina. An optimum combination of activity and stability has been difficult to achieve.
Alumina-boria catalyst composites are known in the art, and in particular were extensively investigated at one time in the catalytic cracking art. However it was in general considered desirable to retain a substantial surface area, above about 150 m.sup.2 /g in the final catalyst composite, and for this reason it was the practice to calcine such catalysts at relatively low temperatures, below about 1200.degree. F., which are below the temperature required for the formation of crystalline aluminum borates. At the other extreme, U.S. Pat. No. 3,172,866 discloses catalyst supports prepared by calcining alumina-boria mixtures containing less than 5 weight-percent boria at temperatures of 1600.degree.-1800.degree. C. (2912.degree.-3272.degree. F.), under which conditions the boria is apparently sublimed out of the composite, and a final alpha alumina support having a surface area below 0.5 m.sup.2 /g is produced.
It has now been found that for purposes of producing a catalyst of maximum activity and stability for high temperature, vapor phase conversions such as exhaust gas conversions, a much superior support is produced by calcining certain alumina-boria composites within the temperature range of about 1250.degree.-2600.degree. F. Calcination within this range appears to produce a definite crystalline phase of 9Al.sub.2 O.sub.3.sup.. 2B.sub.2 O.sub.3 and also in most cases a crystalline phase of 2Al.sub.2 O.sub.3.sup. . B.sub.2 O.sub.3. Although calcining such composites at temperatures below or above the specified range can produce supports of adequate stability for some purposes, it appears that within the range of about 1250.degree.- 2600.degree. F., an optimum combination of crystallinity, porosity, surface area, and/or chemical properties is produced, such that a distinct maximum activity is achieved from active metals supported on such supports. Also, such catalysts exhibit excellent thermal and mechanical stability up to temperatures of about 2500.degree.- 3000.degree. F., depending mainly upon the type of active metals present. They are also highly resistant to shrinkage at temperatures up to at least about 2500.degree. F.